1. Technical Field
The present invention relates generally to integrated circuit internal package interconnects, and more particularly, to a methodology and multi-layer substrate that has improved signal integrity and impedance matching.
2. Description of the Related Art
High-density interconnect schemes for processor packages, as well as other very-large-scale integrated (VLSI) circuits typically use a large number of circuit layers to connect one or more dies to electrical terminals disposed on one or more surfaces of the package, as well as to interconnect multiple dies in multi-die packages.
A typical stack-up for a present-day VLSI circuit substrate is fabricated in very thin layers on one or both sides of a rigid core that provides stiffness and stability to integrated circuit substrates, which may then be encapsulated after dies are attached. The core typically includes pass-through vias that have a larger diameter than the vias used between the thin circuit layers and that pass between thin insulating layers. For example, in a substrate having a core 800 μm thick, the diameter of the through vias may be 500 μm in diameter, while the outer layer interconnects may have vias only 50 μm in diameter. The reason for the larger diameter holes through the core is the relative thickness of the core, which makes reliable fabrication and resin/conductive filling of the vias more difficult than for vias between the thin insulating layers in the outer circuit layers that are laminated on the core.
Since the interconnect routing density directly determines the required size of the final package, routing resources are critical in an integrated circuit package and space is at a premium. However, for critical signal paths such as clock and high-speed logic signal distribution, transmission lines must be maintained throughout the signal path in order to prevent signal degradation. Therefore, a reference voltage plane (e.g., ground) metal layer is provided on the surface of the core, with voids around the via and interconnect areas at the surface(s) of the core so that a transmission line is provided for the next signal layer above/below the core surface metal layer(s). As a result, signal path conductors must be routed around the large diameter vias passing through the core which are not connected to the metal layer. Further, the signal path conductors must also be routed away from discontinuities in the metal layers(s) caused by the voids through which the vias pass, since the lack of reference voltage plane metal will cause a change in impedance of the transmission line. Therefore, the number of signal routing channels is severely limited by the presence of the large-diameter vias that extend through the core that provide signal paths, and the large-diameter vias that provide voltage planes other than the voltage plane connected to the core surface metal layer.
It is therefore desirable to provide a multi-layer integrated circuit, substrate and method that maintain signal integrity and impedance matching in an integrated circuit package while providing an increased amount of signal routing channels.